Rotary piston engines of the type for which the side seal of the present invention is intended have a multi-lobed piston rotatably mounted on a crankpin of a crankshaft, with the crankshaft rotatably mounted in a housing. The multi-lobed piston has the shape of a trochoid or of a curve inside and parallel to a trochoid, while the housing has an inner surface shape corresponding respectively to the outer enclosing curve, or outer envelope, of the trochoid or of the inside parallel curve, these alternatives hereinafter being encompassed by reference to "trochoid" or the "outer enclosing curve, or envelope" thereof. Rotational phasing of the piston relative to the shaft and the housing is ensured by an external gear which is fixed to the piston end which meshes with an internal gear fixed to the housing. The trochoid has at least two lobes and, theoretically it can have any greater number of lobes. The number of lobes determines the gear ratio required for phasing and also the number of working chambers in the housing; the number of such chambers being equal to the number of lobes plus one. Thus a family of trochoidal piston rotary engines exists. The present invention is particularly relevant to a two lobe trochoid rotary piston engine with three working chambers, and the following description largely is directed to two lobes, trochoid shaped piston members. However, the invention extends to engines having more than one piston member and to engines having at least one piston with three or more lobes.
In such rotary piston engines, each working chamber defines or has an associated combustion chamber. It is well known that a major problem is that of sealing the working chambers. This is required so that hot combustion gases do not escape through small but necessary running clearances between each side face of the piston and an adjacent side plate surface of the engine housing, but rather discharge through opening ports or valves. In general, rotary engines have peripheral or apex seals, usually mounted in the housing at apexes, to seal between adjacent working chambers at the outer, peripheral seal of the piston. The engines also have a respective side seal located in a peripheral groove in each side face of the piston, and resiliently biased to rub against and conform to the adjacent side plate surface. The side seal also is intended to conform to a surface of the groove under prevailing gas pressure.
A side seal usually is in the form of an elongate ribbon of rectangular cross-section, and curved and presented side-ways into the groove. The side seal is relatively thin to provide low seal friction with the surface of a side plate against which it is biased for sealing engagement, and has a width several times its thickness. Each seal generally is a relatively neat fit within its groove in the piston side face. A longitudinal spring usually is provided in the groove, between the bottom of the groove and the inner edge of the seal, to ensure that the seal is biased against the adjacent side plate surface. Gas pressure, communicated to the inner edge of the seal, provides an additional force acting to urge the seal into sealing engagement with the side plate surface.
The side seal groove in the piston must be positioned sufficiently far inside of the outer periphery or profile of the piston to ensure that a resultant cantilevered land section of the piston, between the groove and the outer periphery of the piston, is wide enough to have adequate structural strength. However, it is also important to keep the width of the land to a minimum. This is because the width of the land and the required running clearance between the piston and the housing side plates define areas, between the side seals and the apex seals, through which gas leakage between adjacent working chambers can occur, even though button seals can be used to minimise the leakage through these areas. In general, it is desirable that the side seal groove is a minimum distance inside of the outer periphery of the piston providing for the structural integrity of the cantilevered land of the piston, and that the groove is curved so as to be substantially parallel to the outer periphery or profile of the piston.
A reciprocating engine relies on compression rings to seal against the bottom of the piston groove as well as against the walls of the cylinder during combustion. Whilst prior art rotary engines allow for the hot compressed gas to get behind the side seals and press the side seals against the side plate surfaces, it can not be ensured that the side seals are pressed against the inner side of their grooves under dynamic conditions. This is particularly so at high speeds, since centrifugal force tends to force the sealing strip outwardly against the combustion pressure. Under these conditions the side seals float between the sides of their grooves and allow gases to escape around them. In addition, the action of rotatory and orbiting motion of the rotary piston tends to act to dislodge the side seals in their trailing ends. This has the consequence of producing a gas leakage path around the flanks of the side seals and thus contributes to low engine performance.
In trochoidal piston rotary engines it has been found in practice that gas pressure is inadequate to force the side seals against the inner side of their grooves. It also is found that substantial sealing against at least one, but preferably each, side of the groove is necessary. It is an object of this invention to provide a side seal enabling this type of sealing.
Specific prior art in producing a seal behind side seals is provided in U.S. Pat. No. 3,881,848 to Huf. In this, a cup-shaped insert is located behind the side seal. However in practice, particularly in applications where the side seal is of small dimensions, of the order of 1 mm thick or smaller, this method is very difficult. In addition, gas is not prevented from reaching the opposite side of the rotor, creating high seal load and preventing the high pressure gas from doing work. Also it does not prevent the gases from escaping along the flanks of the outer side seal.